1. Field of the Invention
The invention relates to a projection device enabling the formation of an image on a distant screen with a high luminous efficiency, through the use of the two orthogonal components of light polarization.
2. Description of the Prior Art
In image projectors, the image projected on a screen results from a spatial modulation of light. The light produced by a source is guided up to a set of light modulators marshalling the means needed to achieve the spatial modulation of light. It is common to use a liquid crystal display (LCD) screen as a spatial light modulator. This LCD screen has a matrix array of elementary liquid crystal cells, controlled by means of a video signal. Each cell represents an elementary dot of the image. The LCD is also called an optical valve or, again, an "electronic slide".
Liquid crystal cells are generally of the twisted nematic type with 90.degree. helix. These liquid crystal cells are the best known ones, and are generally used in thin film transistor technology enabling them to be addressed by a video signal. This means that each cell is controlled by means of a switching element or TFT (thin film transistor).
In the resting state, each elementary cell of the LCD has a capacity of rotation (by 90.degree.) in two orthogonal directions of polarization of the light that goes through it. When an elementary cell is activated, i.e. when the electrical field applied to a cell increases, under the effect of the video signal for example, the rotational capacity gradually disappears. Rotational angles of 0.degree. to 90.degree. may be obtained by causing a continuous variation in the signal applied to the "TFT", i.e. to the elementary liquid crystal cell.
On the basis of this principle, the working of a standard image projector using a spatial modulator such as an LCD requires the selection of a direction of polarization for the illumination of the LCD screen, so that the complementary polarization is not used. To preserve only one direction of polarization, the light goes through a polarizer before illuminating the LCD screen.
After passing through the LCD screen, the light may again have two complementary types of polarization according to a spatial distribution that depends on the cells and on the degree of their activation. The display of the state of each cell of the LCD screen is obtained by making the light that emerges from the LCD screen pass through a means for the analysis of polarization, a polarizer for example.
The method explained here above is the one most commonly used, because it is the one that is least complicated to implement. However, it has the major drawback of leading to a low luminous efficiency. Indeed, the removal of one of the components of light with a given polarization directly leads to the loss of at least 50% of the light energy, plus the absorption by the polarizer and the analyzer of the light having the useful polarization direction.